add support for token validation on login page

lib/services/authservice.dart

@@ -72,8 +72,8 @@ class AuthService extends ChangeNotifier {
 
   Future<void> tryRestoreSession() async {
     if (_restoring || _user != null) return;
-      _restoring = true;
-      try {
+    _restoring = true;
+    try {
       // read token from secure storage (with fallback)
       final token = await _tokenStorage.getToken();
       if (token == null || token.isEmpty) return;
@@ -100,6 +100,24 @@ class AuthService extends ChangeNotifier {
     }
   }
 
+  Future<bool> validateStoredToken() async {
+    final token = await _tokenStorage.getToken();
+    if (token == null || token.isEmpty) return false;
+    try {
+      await api.get(
+        '/validate',
+        headers: {
+          'Authorization': 'Bearer $token',
+          'accept': 'application/json',
+        },
+      );
+      return true;
+    } catch (_) {
+      await _clearToken();
+      return false;
+    }
+  }
+
   Future<void> _persistToken(String token) async {
     await _tokenStorage.setToken(token);
   }